There is a known analyzing method called a Fourier transform spectrum analysis (see Patent Document 1, for example). The Michelson interferometer is known as an optical system related to the Fourier transform spectrum analysis.
In an interferometer for a Fourier transform spectrum analyzer, light from a light source is separated into two optical paths by a beam splitter, and the separated two light beams are then reflected by mirrors disposed in the optical paths and are combined by a beam splitter.
In such interferometer, a light source having a wide emission spectrum including infrared light is used; one separated light beam in the interferometer is reflected by a traveling mirror (moving mirror); the other separated light beam is reflected by a static mirror (fixed mirror); whereby an optical path difference is given between the separated two optical paths; and a frequency spectrum included is obtained by Fourier transforming an intensity profile (interferogram) from the light source obtained by receiving the combined light (interfering light).
By spectrum analyzing a transmitted light having passed through an analysis sample by using a Fourier transform spectrum analyzer, the spectrum absorbed in the analysis sample is obtained.
In recent years, there have been developed small sized MEMS mirrors that move while keeping parallelism, and by using such mirror as a moving mirror, a Fourier spectrometer has been made smaller.
In order to realize high wavelength resolution, a Fourier spectrometer is configured to have a long optical path difference between the two optical paths. In addition, in order to obtained high accuracy, the optical path difference is measured by laser light. In particular, a helium-neon laser having a long coherence length is used, and an interferometer having an optical path of the laser light is installed on an optical path different from an infrared light interferometer having an optical path of the infrared light (measurement light beam), and thus the optical path difference is measured from the intervals between the interference fringes observed in association with the movement of the moving mirror (see Patent Document 2, for example).